Industrial Waste Used in Floor Tile Glazes

Size: px

Start display at page:

Download "Industrial Waste Used in Floor Tile Glazes"

Tobias Price
5 years ago
Views:

1 Fig. 6. Response surface of studied properties. Industrial Waste Used in Floor Tile Glazes An attempt was made to reuse ceramic waste as a floor tile glaze via two-charge and double-pressing dry-glazing processes. F.J.P. Sousa, A.P. Novaes de Oliveira and O.E. Alarcon One of society s biggest problems is the rational management of its waste. Therefore, the use of waste as a raw material seems to be a technically and an economically viable alternative. Reuse of waste decreases environmental damages and leads to decreased exploitation of natural sources. 1 Specifically, a small amount of glaze waste is generated in dry-glazing processes, 2,3 which decreases environment damage. It has become necessary to verify the technical viability of the reuse of industrial waste that is generated by the Cecrisa Co. Portinari Plant (Criciúma City, Santa Catarina, Brazil) for glazing ceramic floor tile. The manufacture of ceramic tile is conducted according to a specific-step process, which requires rigid control of involved variables, in order to obtain commercial products that meet technical specifications. The success of the glazing process is important, because the thin glaze layer is the surface that is observed after it is fixed to the ceramic tile body. The glazed surface, especially in case of ceramic floor tile, is susceptible to chemical attack and abrasive wear. The glaze layer provides properties and characteristics of hardness, abrasive-wear and stain resistance, impermeability, facility of cleaning and aesthetic aspects. 4 The aesthetic aspect may require extra steps of decoration and firing. Glaze Application The application of the glaze layer generally is accomplished in glazing lines. These lines constitute belts on which the dried piece is placed and conducted to a glaze application machine, e.g., a continuous slip veil. A slip is a carefully prepared suspension that transports ceramic powders. 5,6 The slip has certain rheological 9101

properties, susch as constant viscosiy, high density, and low shear stress and thixotropy. 5 7 The ceramic green body has suitable porosity to control water absorption from the slip.

2 properties, susch as constant viscosiy, high density, and low shear stress and thixotropy. 5 7 The ceramic green body has suitable porosity to control water absorption from the slip. The glaze and ceramic body must be chemically and thermophysically compatible. 7 A glaze also can be applied to the ceramic piece using a dry process. The two-charge technique, 2,3 which uses granulated Temperature ( C) glazes, is such a process. Two Fig. 2. Vitrification diagram for the glaze body pair. charges of granulated glazes constitute the die fill. Therefore, two layers directly bonded to each other are formed. One layer is compounded by granulated glazes and the other layer by a granulated ceramic body that forms the substrate. In both cases, granules are obtained from a spray dryer. The layers are compacted and remain mechanically adhered after they are extracted from the press. Amoròs 7 used two-charge technology during the 1970s. However, because of its limited aesthetic effects, two-charge technology has been little used. Moreover, the two-charge technology requires extra devices, because two charges are executed and each requires homogeneous die filling. However, Fig. 1. Sketch of the die used to manufacture samples. Cantavella et al. 8 have reported obtaining aesthetic effects in the production of porcelainized stoneware tile using combined successive charges. The use of this process has been on the increase during the past 10 years in the Spanish market. The two-charge process also can be used as an economic way to manufacture more noble ceramic bodies. The resulting ceramic tile has two layers, whereas inferior products are formed using a ceramic body without pigment additions. Another form of the two-charge process is double pressing. 9 Double pressing has two steps. The first is to compact the substrate granules so that the free volume in the die can be filled by a granulated glaze. The second is to compact both layers by applying a selected (final) pressure. For a given die depth, the pressure used in the first compaction governs the glaze layer thickness. Both two-charge processes, especially the double-pressing process, have lower productivity than the wet-glazing process. However, the two-charge processes eliminate glazing lines. 7 Glazing-lines require considerable physical space, frequent adjustments and adaptation of belts, sensors, and other devices for production of lots with even slightly different specifications. It has become necessary to study the first compaction effect on glaze adherence to the substrate and the parameters for a suitable firing cycle. Water absorption, four-point bending strength, abrasion wear and hardness needed to be determined. Hardness was determined using a scratch test. A scratch was created on the surface of the glazed piece using controlled load and speed. The average scratch width was used to obtain a hardness value, including dynamic effects. High scratch hardness values are related to glazed ceramic floor tile with high abrasion wear resistance. 10,11 The waste used in this work resulted from processing steps in the production of ceramic tile and washing of equipment. Since the waste contained much material from the glazing step, it was decided to use the waste as a glaze. Dry-glazing technology was used because of the relative facility of sample preparation, and because the dry processes did not require glazing lines. Materials Used Value (%) The waste disposed by Portinari Co. was collected and washed in water to remove light organic materials. The resulting material was dried at 110 C to constant weight and then milled to a particle size of <77 µm. Particle size was determined using a laser-scattering particle-size analyzer (Model 1064L, Cilas). Chemical composition of the waste (Table 1) was determined using X-ray fluorescence spectroscopy (Model PW 2400, Philips, 9102

3 Industrial Waste (a) Eindhoven, Netherlands). Further details about waste treatment and characterization can be found in previous works. 1,12 A porcelainized stoneware pressed at 31 MPa, provided by Portinari, was used as the tile substrate. Three pressure levels were used for the first Fig. 3. Details of the samples: (a) interfacial defects and (b) tested set. compaction: 10, 15 and 20 MPa. These levels corresponded to about 1/3, 1/2 and 2/3 of the final compaction pressure. A glaze/substrate vitrification diagram was used to select a suitable firing temperature. Samples were fired in a laboratory furnace (Nannetti) at a heating rate of 15 C/min with holding times of 5, 10 and 15 min at selected temperatures. Granulated powders with moisture content of ~6% were pressed in a laboratory hydraulic press (Figure 1). Nominal sample size was mm, which depended on the die used. Double-Pressing Processes The double-pressing process was investigated initially. The process consisted of applying a given (first) compaction pressure to the ceramic body (substrate). The die subsequently was filled with granulated glaze and final compaction pressure applied. When the ceramic body was compacted at higher pressures, the available height in the die for the second charging was greater. Therefore, more granulated glaze was necessary to fill the die. When the doublepressing glazing process was used, a height adjuster device controlled die filling. First, ceramic body granules were added to the die to the maximum die level. Second, excess granules were removed using a metallic ruler. Third, the die was carefully suspended and remained at ~4 mm using a metallic support. The compacted ceramic body granules provided space in the die cavity for glaze granules. Each compaction consumed ~4 g of glaze and 20 g of ceramic body. The number of visually verified defects in each produced lot was used to determine the success of the process. Conformity with NBR 13818, 13 which is related to water absorption (WA) and four-point modulus of rupture (MOR), was adopted. The Table 4 Results of Factorial Design for WA WA factor SS DF MS F calc p-value First compaction pressure (1) Holding time (2) (1) by (2) interaction Error Total SS Red-highlighted lines indicate factors with influence over the dependent variables. Table 5 Results of Factorial Design for MOR MOR factor SS DF MS F calc p-value First compaction pressure (1) Holding time (2) (1) by (2) interaction Error Total SS Red-highlighted lines indicate factors with influence over the dependent variables. (b) four-point MOR test was adopted because it requires higher sample volume and it does not introduce shear stresses in the sample rupture region. MOR tests were conducted according to ARMC-TR Final samples were fast-fired in a laboratory furnace (Schaly) for 2 h. Results analysis of various lots was made using factorial design statistical tests that separately considered each property (WA and MOR). This confirmed the superiority of a lot with respect to others with a confidence level of 10%. The method also allowed study of the interaction between variables. 9103

4 MOR (MPa) Water absorption (%) First compaction pressure (MPa) First compaction pressure (MPa) Fig. 5 Illustration of the factors interaction. Random positioning of samples inside the furnace minimized thermal gradient effects and provided important requirements related to statistical tests 15 (the randomness of variables). Normal distribution and homogeneity between the respective variances also needed to be satisfied. The variances needed to be of the same magnitude order. The normality of the distribution of each sample lot was represented by the correlation coefficient (R 2 ) of the respective deviations. Abrasion wear tests (PEI) and scratch hardness measurements of selected samples were made. The scratch hardness was determined using a sclerometer. A pyramidal diamond indenter was used. The applied normal load was 0.5 N, and the scratching speed was 2 mm/s in a test line of 6 mm length. The average scratch width was calculated using scanning electron microscopy (SEM) images. Only ductile scratches in randomly selected points were considered. Test Results Vitrification diagrams of the samples were prepared (Figure 2). Each point plotted was the average of two measurements of WA and linear shrinkage. The vitrification diagram curves suggested that the suitable firing temperature was ~1200 C. This took into consideration the highest shrinkage value and, simultaneously, the smallest WA value. This temperature coincided with that used by the industry. After the samples were fired, they were submitted to visual inspection to verify interfacial defects (Figure 3). Samples from all batches then were submitted to the four-point bending test (Table 2). The defect frequency within the batch pressed at 20 MPa (first compaction level) indicated that this pressure was too high. These defects probably were caused by the lack of mechanical bonding between substrate and glaze particles. In other words, a high first compaction level of the ceramic body made it difficult for penetration of glaze particles at the layer interface. Therefore, lower levels of first compaction needed to be adopted. Other sample batches achieved Brazilian Standard NBR They achieved WA values <0.5% and mean MOR values >35 MPa. Moreover, the lowest MOR value was >32 MPa. Thus, only the sample batches with first compaction levels of 10 and 15 MPa were submitted to a firing cycle with a holding time of 10 min (Table 3). All batches pressed at 10 and 15 MPa exhibited technically acceptable results. However, interfacial defects occurred in some batches pressed at 15 MPa for 5 or 10 min holding time. The results were regarded as faulty samples and were not included as part of the statistical parameters. Therefore, five replications were admitted. Interfacial defects also could be identified by WA because of the water lodged inside the interfacial defects (Table 2). However, samples with interfacial defects did not cause decreased MOR values. This behavior was attributed to the use of the four-point bending test. The constant moment generated at the rupture region caused no shearing stress. Therefore, no interfacial adherence was required. 9104

Industrial Waste Fig. 7. Scratch produced by pyramidal indenter. Statistical Tests Factorial design was used to compare the batches prepared under various pressing and firing conditions (Figure 4).

The p-values were assessed from an F-curve. They represented the probability of committing a mistake on affirming that each variable had influence over the output property.

5 Industrial Waste Fig. 7. Scratch produced by pyramidal indenter. Statistical Tests Factorial design was used to compare the batches prepared under various pressing and firing conditions (Figure 4). Two factors were considered for the factorial design: compaction level and holding time, each at two levels. Output proprieties were WA and MOR of each batch (Tables 4 and 5). The p-values were assessed from an F-curve. They represented the probability of committing a mistake on affirming that each variable had influence over the output property. Regarding interaction effects, a small p-value suggested a reciprocal influence between these input variables. The sum of square (SS), degree of freedom (DF) and F calc values were calculated using Microsoft EXCEL and STATÍSTICA software. The p-values were compared with the confidence level adopted (0.10). The result verified that the holding time played a positive role in WA of the samples. However, the first compaction pressure caused no significant effect, because the p-value was >0.10. Moreover, it seemed that only the holding time directly affected the MOR behavior of the samples. An interaction between the input variables also was observed for the WA. Because of this interaction, the condition related to the smaller first compaction levels and longer holding times led to a significant decrease in WA. Such an interaction meant that the effect of the holding time was different according to the first compaction level used (Figure 5). This interaction occurred because, when the level of previous compaction was lower, the interchange between powders from glaze and ceramic body layers was easier. This made no difference for short holding times. However, for longer holding times at a given firing temperature, interpenetration became important. The influence of each factor on WA and MOR results was studied (Figure 6). Fig. 8. Superficial porosity of glaze layer. The statistical tests showed that the best manufacturing condition was 10 MPa for the first compaction level and 10 min for firing holding time. Thus, one sample of this batch was used for the scratch test. Afterward, another sample was manufactured using the same conditions of this batch. However, it had final dimensions suitable to the PEI test to complement the abrasion-wear characterization. SEM showed that the scratch promoted on the glaze surface revealed high porosity on the glaze surface (Figures 7 and 8). Ductile tracks were required to estimate the scratch hardness. Therefore, a low normal load of ~0.5 N was applied. The results related to the studied glaze were compared with other commercial products (Table 6). 1 The glaze studied exhibited poor scratch hardness, even when compared with commercial glass. The surface abrasion-wear test classified the glaze as PEI 2. Therefore, the abrasion-wear resistance of the glaze was considered fair. When the two-charge process was used (i.e., where first compaction does not exist), the only holding time adopted was 10 min, and sample linear shrinking was determined (Table 7). The higher values of WA compared with those achieved using the double-pressing process were explained by the sample green density gradient. The extra thickness caused by the addiction of the glaze layer without previous pressing of the substrate increased the sample thickness/base ratio. This led to a higher internal friction between the powders and the die walls. This, in turn, caused expressive gradients of green density through the bulk of the sample. Such an effect was confirmed by the correlation between the final properties described above and the green sample weight, because, the higher the apparent weight, the higher the sample thickness. The R 2 values obtained for the MOR and WA were 0.94 and 0.97, respectively. These negative high values for R 2 mean that, the greater sample thicknesses, the smaller the studied properties. Therefore, a more suitable die design was tested, i.e., one with a lower die depth/area ratio. Another six 9105

6 samples then were manufactured using a die with nominal dimensions of mm. Therefore, the sample thickness depended on the amount of added powder. As expected, the mean WA value was ~0.24%, which was less than those samples previously tested. On the other hand, after the firing step, the selected sample dimensions resulted in thermal expansion incompatibility between the glaze and the ceramic body, and warping was evidenced (Figure 9). The glaze layer was subjected to a prescribed tensile stress. Therefore, any bending test selected would be biased. Waste Cannot Be Used Alone The thermophysical incompatibility, poor scratching hardness and wear resistance, and high superficial porosity of the glaze strongly suggests that the waste studied cannot be used as ceramic glaze unless other raw materials or additives are used. On the other hand, all these poor properties are inherent to the waste material. Therefore, the use of a dry-glazing process, either double-pressing or two-charge, is recommended because of the acceptable results of flawless samples. Others formulations that contain less waste should be tested that maintain the option of either doublepressing or two-charge glazing processes. Flawless samples can be achieved using either process. Acknowledgments The authors are grateful to CAPES and CNPq/Brazil for funding this work as well as the interest of Cecrisa/Portinari Co. for attempting to develop products with an ecological point of view. About the Authors The authors are faculty members of the Dept. of Mechanical Engineering, Graduate Program in Materials Science and Engineering, Federal University of Santa Catarina, Florianópolis, S.C., Brazil References 1 F.J.P. Sousa, Estudo e Desenvolvimento de Alternativas Para o Aproveitamento de Resíduo das Indústrias de Revestimentos Cerâmicos ; M.S. Dissertation. Dept. Engenharia Mecânica, Universidade Federal de Santa Catarina, Florianópolis, Brazil. 2 J.L. Amorós, Vidrados Para Pavimentos e Revestimentos Cerâmicos: Evolução e Perspectivas. Parte II, Ceram. Ind., [6] (2001). 3 G.P. Emiliani and F. Corbara, Tecnologia Ceramica: La Lavorazione, Vol. 2; p Faenza Editrice, Modena, Italy, S. Stefano and B. Svetlan, Smalti Ceramici; p Faenza Editrice, Traduzione di Vittorio Emiliani, Modena, Italy, A.P.N. de Oliveira, Tecnologia de Fabricação de Revestimentos Cerâmicos, Ceram. Ind., [6] (2000). 6 Società Ceramica Italiana (Eds.), Reologia Ceramica Aplicata; p Faenza Editrice, Modena, Italy, J.L. Amorós, Vidrados Para Pavimentos e Revestimentos Cerâmicos: Evolução e Perspectivas. Parte I, Ceram. Ind., [4] (2001). 8 Cantavella, E. Sànchez, J. García-Tem, C. Feliu, J. Sánchez, C. Soler, J. Portolés and J. Sales, Residual Stresses in Porcelain Tiles Formed by Two-Charge Pressing, Tile Brick Int., 16 [5] (2000). 9 A. Bresciani and C. Ricci, Innovative Process for Ceramic Tile Manufacture by Double Pressing with Continuous Precompactation ; pp in QUALICER Castellón, R.E.F.Q. Nogueira and J.B.D. Mello, Abrasão do Quartzo: Um Estudo Esclerométrico ; p in Proceedings of Anais do VIII Congresso Brasileiro de Ciência e Engenharia de Materiais CBECIMAT (Campinas, 1988). 11 O.E. Alarcon, W.L. Weingaerner, H.R. Roman, F.J.P. Sousa, M. Pereira, A. Spiller and D. Tridapalli, Design and Development of an Apparatus to Measure the Scratch Hardness of Ceramic Tile ; in QUALICER Castellón, F.J.P. Sousa, W.F. das Neves and O.E. Alarcon, Estudo da Viabilidade de Obtenção de Isoladores Elétricos a Partir de Resíduo de Esmaltação, Ceramica, 50, (2004). 13 Brazilian Association for Technical Standards, NBR 13818, Placas Cerâmicas Para Revestimento Especificação e Métodos de Ensaios, Rio de Janeiro, Army Material Research Center, Tech. Rept. AMRC TR 8220, Mass., April D.C. Montgomery, Design and Analysis of Experiments, 4th Ed. Wiley, New York,

Table 1 Chemical Analysis of Waste Oxide Waste (wt%) SiO 2 56.95 Al 2 O 3 15.05 B 2 O 3 Fe 2 O 3 1.89 CaO 4.89 Na 2 O 1.03 K 2 O 2.22 MnO 0.03 TiO 2 0.60 MgO 1.79 P 2 O 5 0.19 ZnO 3.

7 Table 1 Chemical Analysis of Waste Oxide Waste (wt%) SiO Al 2 O B 2 O 3 Fe 2 O CaO 4.89 Na 2 O 1.03 K 2 O 2.22 MnO 0.03 TiO MgO 1.79 P 2 O ZnO 3.32 PbO BaO ZrO LOI 8.03 LOI is loss on ignition. Table 6 Results of Scratch Test Normal load Scratching Material tested (N) hardness (GPa) Glaze processed from solid wastes Soda-lime commercial glass Porcelain stoneware tile Table 2 Results Using 5 min Holding Time Table 3 Results Using 10 min Holding Time First compaction level 10 MPa 15 MPa WA MOR WA MOR Sample (%) (MPa) Flaw (%) (MPa) Flaw X σ R Indicates flaw. First compaction level 10 MPa 15 MPa 20 MPa WA MOR WA MOR WA MOR Sample (%) (MPa) Flaw (%) (MPa) Flaw (%) (MPa) Flaw X σ Indicates flaw. Table 7 Results of Two-Charge Process Sample Green sample (g) Weight loss (%) WA (%) Linear shrinkage (%) MOR (MPa) X σ Previous compaction Threshold time Fig. 4. Factorial design established (2 2 ). Fig. 9. Warping of platelike samples after firing.

Crystallization Mechanism and Kinetics of BaO-Li2O-ZrO2-SiO2 Glasses

Materials Research, Vol. 5, No. 1, 21-26, 2002. 2002 Crystallization Mechanism and Kinetics of BaO-Li2O-ZrO2-SiO2 Glasses Cristian Berto da Silveira a, Sílvia Denofre de Campos a, Elvio A. de Campos a,